| This study was carried out for Larix principis-rupprechtii plantations with the same age of 25 years,the same site quality,but different tree densities,growing in the Xiangshuihe small watershed of the southern part of Liupan Mountain in Ningxia.The structure and growth characters of forest trees and plant community under canopy were determined in selected stands with densities of 800,1000,1200,1500,1800 trees/hm2.The hydrological effects of the acting layers of tree canopy,humus,and root-zone soil were also measured and compared.Finally,the multiple eco-hydrological effects of plantation were integratedly assessed through the principal component analysis (PCA).The rational demsity for the stands studied was determined based on the principle of maximizing the eco-hydrological service.This study is helful in practice for determining the objective density of stand.The main conclusions are as follows:(1) The structure of forest stands.The tree diameter (DBH) was fitted using Weibull distribution function,and it was shown that stands with density of 800,1000 and 1500 trees/hm2 match the Weibull distribution function.Within the stands with different densities,but under the same conditions of age and site quality,the DBH distribution curve varies with the forest density.The DBH is bigger in the stand with lower tree densities,and smaller in the stands with higher densities.The DBH distribution presentes as single-apex–curve,but leaning more left with increasing tree density.While the tree height distribution shows a single- or double-peak-curve pattern.The tree height is positively related with the DBH,i.e.,the tree height increases with increasing DBH.(2) Biodiversity of vegetation under tree canopy.The plant species diversity and richness of herb layer is obviously higher than those of shrub layer.When the forest density is 800 trees/hm2,the height and coverage of shrubs arrive at their maximum value,of 138.8 cm and 25.3%.When the forest density is 800 and 1000 trees/hm2,the herb layer height and coverage arrive their maximum value of 46.7cm and 71.6%.When the forest density is 1800 trees/hm2 and1500 trees/hm2,the species richness of both herbs and shrubs reaches its maximum of 10 and 28.In the stand with density of 1500 trees/hm2,the indexes of richness of herbs and shrubs,and the herb layer indexes of Simpson,Shannon-Wienerand,and the uniformity index are the maximum,with the value of 2.216,1.909,0.898,2.565,0.842 respectively.(3) Hydrological effects of tree canopy.The quantity of stemflow,throughfall and canopy interception is a linearly related with the rainfall amount in open field.The stemflow ratio,throughfall ratio varies with the rainfall depth in a logarithmic function. During the study period from May to October in 2011,the rainfall depth in open filed was 813.0 mm.the order of throughfall depth follows the order of stands with densities of 800,1800,1000,1200,1500 trees/hm2,their throughfall was 654.08,636.42,624.86, 617.88,613.06 mm,respectively.The canopy interception varies with rainfall depth in a logarithmic function,while the canopy interception ratio decreases with rainfall depth in a power function.The order of canopy interception follows the order of stands with density of 1500,1200,1000,1800,800 trees/hm2,and their canopy interception depth was 197.51,192.89,185.77,173.92,156.90 mm,respectively.The interception depth increased firstly with increasing tress density and then decreased when the tree density further increases;while the throughfall varies in an opposite trend with increasing tree density. This can be explained by the fact that the tree density is not the directly influencing factors,rather the LAI directly influences the throughfall and canopy interception. Therefore,the variation of canopy interception and throughfall was more related or more in consistant with the canopy LAI.The canopy interception and LAI were the highest in the stand with a density of 1500 trees/hm2.(4) The hydrological effects of humus layer.The weight of humus layer in the plantation stands with different densities varies in the range of 8.12-12.50 t/hm2.The accumulated water soaked by the humus materials showed an increasewith the soaking time, following a logarithmic curve (S=alnt+b);while the absorption rate decreased with the soaking time,following a power function curve(V=atb).Among the stands with different densities,the order of water absorbing rate of humus layer materials follows the order of density of 1500>1200>1800>1000>800 trees/hm2.Within the same stand with a fixed density,the water absorbing rate of half-decomposed litter was higher than that of the un-decomposed litters.The amount of humus materials and their water-holding-capacity,water absorbing rate and effective water absorption reach the highest values when the stand density is 1500 trees/hm2。(5) The hydrological and physical properties of root-zone soil layer.The bulk density of soil increases with increasing soil depth under all stands with different densities.The average bulk density of the 0-100 cm soil layer increased firstly with stand density and then decreased with the further increase of stand density,following the order of stand density of 1200,1000,1800,800 and 1500 trss/hm2,with the corresponding bulk density of 1.420,1.176,1.168,1.141 and 1.111 g/cm3,respectively.The capillary porosity,total porosity,the water-holding capacity and the water holded in capillary pores decreased with increasing soil depth under all forest stands studied.When the forest density is 1500 trees/hm2,the capillary porosity,total porosity,non-capillary porosity and the average of water holding capacity reached the maximum.The effective water-holding capacity of the 0-100 cm soil layer of the plantation stands with different densities varies in the range of 96.59-184.41 mm,showing a stand density order of 1500>1800>800>1000>1200 trees/hm2.The water storage function of soil layers varies with forest density.The highest effective water-holding capacity of 120.33 mm appears in the forest stand with density of 1500 trees/hm2,while the lowest effective water-holding capacity of 62.79 mm appears in the stand with density of 1800 trees/hm2.(6) Integrated assessment of the eco-hydrological effects of stand density.Under the condition of same site quality and tree age of 25 years,the Larix principis-rupprechtii plantation with the density of 1500 trees/hm2 presents the best ecohysrological effectss, and the stand with a density of 1200 trees/hm2 takes the second best position,while the stand with density of 800 trees/hm2 has the worest ecohydrological effects. |
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